MdeModulePkg: PiDxeS3BootScriptLib: honor PcdAcpiS3Enable

MdeModulePkg: PiDxeS3BootScriptLib: honor PcdAcpiS3Enable

In the edk2 tree, there are currently four drivers that consume
PcdAcpiS3Enable:

  IntelFrameworkModulePkg/Universal/Acpi/AcpiS3SaveDxe/AcpiS3SaveDxe.inf
  MdeModulePkg/Universal/Acpi/BootScriptExecutorDxe/BootScriptExecutorDxe.inf
  MdeModulePkg/Universal/Acpi/S3SaveStateDxe/S3SaveStateDxe.inf
  MdeModulePkg/Universal/Acpi/SmmS3SaveState/SmmS3SaveState.inf

From these, AcpiS3SaveDxe is the only one that isn't also a client of the
S3BootScriptLib class; all the others (BootScriptExecutorDxe,
S3SaveStateDxe, SmmS3SaveState) are clients of the S3BootScriptLib class.

In turn, the edk2 tree contains only one non-Null instance of the
S3BootScriptLib class:

  MdeModulePkg/Library/PiDxeS3BootScriptLib/DxeS3BootScriptLib.inf

Therefore we can safely state that BootScriptExecutorDxe, S3SaveStateDxe,
and SmmS3SaveState are all linked against PiDxeS3BootScriptLib.

Now, if PcdAcpiS3Enable is FALSE when either of BootScriptExecutorDxe,
SmmS3SaveState, or SmmS3SaveState is dispatched, then the following
happens:

- The constructor of PiDxeS3BootScriptLib, function
  S3BootScriptLibInitialize(), registers a protocol installation callback
  for gEfiDxeSmmReadyToLockProtocolGuid. Namely, the function
  S3BootScriptEventCallBack().

- The driver immediately exits with EFI_UNSUPPORTED from its entry point
  function, upon seeing PcdAcpiS3Enable == FALSE. (See commits
  800c02fbe2da6, 125e093876414, and d2d38610603f6.)

- This leaves a dangling callback pointer in the DXE core.

- When Platform BDS installs gEfiDxeSmmReadyToLockProtocolGuid (which is a
  valid thing to do for locking down SMM, even in the absence of S3
  support!), things blow up.

Fix this issue by returning immediately from S3BootScriptLibInitialize()
if PcdAcpiS3Enable is FALSE -- it is useless to initialize the library
instance if the containing driver module exits first thing in its entry
point.

Cc: Feng Tian <feng.tian@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>